Greenhouses are poorly insulated structures so heat loss through the covering on cold, clear, windy nights can be considerable. Heat can be supplied using electricity (can be expensive), natural gas or propane, fuel oil, solar energy (unreliable), or kerosene (emergency use only). You can also connect the greenhouse to your home heating system. Natural gas or propane are probably the most cost-effective ways of heating a hobby greenhouse. If natural gas is available in the home, plumbing into the existing line saves considerable cost over a new meter and gas line. If natural gas is not available, check with the local propane gas company for cost and availability. Determine if the company provides a storage tank free or if it can be rented or purchased. Consult the company to determine the tank size appropriate for the greenhouse.
A variety of gas unit heaters are available to heat hobby greenhouses. Some are designed to hang from the structure of the greenhouse; others sit on the floor. Costs for gas unit heaters range from $300 to $550 for 20,000 to 60,000 BTU units, respectively. Unit heaters burn gas in a firebox. Heated air rises through the inside of a thin-walled heat exchanger on the way to the exhaust chimney. A fan draws air in from the greenhouse, across the outside of the heat exchanger and out into the greenhouse. Thus, most of the heat is removed from the exhaust before it exits the structure. The exhaust chimney must be sufficiently tall to maintain an upward draft and extend above the greenhouse roof. An 8- to 12-foot chimney is usually sufficient. All open-flame heaters must be vented to the outside and given a fresh air supply for complete combustion. Fresh air should be provided by an unobstructed opening to avoid carbon dioxide buildup.
In larger greenhouses, a plastic tube system may be needed to distribute the heat evenly within the house. The system consists of a perforated polyethylene tube suspended overhead in the ridge and extending the length of the greenhouse. A fan connected to the tube blows warm air from the heater into the tube for greenhouse distribution. This system can also be used for circulating internal air when heating or cooling are not required.
A variety of electric resistance heaters can also be used. Costs range from $100 to $350 for 5,000 to 17,000 BTU units, respectively. Those available in department stores and home centers are usually only adequate in the smallest greenhouse for starting seedlings in the spring. Larger units can be purchased, but operating them can be costly.
Heater size for a given greenhouse and geographic location depends on the surface area of the greenhouse and the temperature difference between the inside and outside of the greenhouse. To determine the size heating system you need, calculate the total surface area of the greenhouse covering. Then determine the difference between the minimum expected outside temperature during winter in your area (see the USDA hardiness zone map, average annual minimum temperature) and the maximum night temperature you wish to maintain (generally 60 to 65 degrees F). Multiply the greenhouse surface area by the temperature difference by the heat loss conversion factor in Table 1 for the appropriate covering. The answer will be in BTUs per hour. Most heaters are rated in this unit. Many greenhouse supply or construction companies can help you determine the proper size heater for your situation.